On Tue, Aug 29, 2023 at 02:32:46PM -0500, Eric Blake wrote: > On Tue, Aug 29, 2023 at 12:06:22PM -0400, Stefan Hajnoczi wrote: > > The ongoing QEMU multi-queue block layer effort makes it possible for > > multiple > > threads to process I/O in parallel. The nbd block driver is not compatible > > with > > the multi-queue block layer yet because QIOChannel cannot be used easily > > from > > coroutines running in multiple threads. This series changes the QIOChannel > > API > > to make that possible. > > > > In the current API, calling qio_channel_attach_aio_context() sets the > > AioContext where qio_channel_yield() installs an fd handler prior to > > yielding: > > > > qio_channel_attach_aio_context(ioc, my_ctx); > > ... > > qio_channel_yield(ioc); // my_ctx is used here > > ... > > qio_channel_detach_aio_context(ioc); > > > > This API design has limitations: reading and writing must be done in the > > same > > AioContext and moving between AioContexts involves a cumbersome sequence of > > API > > calls that is not suitable for doing on a per-request basis. > > > > There is no fundamental reason why a QIOChannel needs to run within the > > same AioContext every time qio_channel_yield() is called. QIOChannel > > only uses the AioContext while inside qio_channel_yield(). The rest of > > the time, QIOChannel is independent of any AioContext. > > > > In the new API, qio_channel_yield() queries the AioContext from the current > > coroutine using qemu_coroutine_get_aio_context(). There is no need to > > explicitly attach/detach AioContexts anymore and > > qio_channel_attach_aio_context() and qio_channel_detach_aio_context() are > > gone. > > One coroutine can read from the QIOChannel while another coroutine writes > > from > > a different AioContext. > > > > This API change allows the nbd block driver to use QIOChannel from any > > thread. > > It's important to keep in mind that the block driver already synchronizes > > QIOChannel access and ensures that two coroutines never read simultaneously > > or > > write simultaneously. > > > > This patch updates all users of qio_channel_attach_aio_context() to the > > new API. Most conversions are simple, but vhost-user-server requires a > > new qemu_coroutine_yield() call to quiesce the vu_client_trip() > > coroutine when not attached to any AioContext. > > > > While the API is has become simpler, there is one wart: QIOChannel has a > > special case for the iohandler AioContext (used for handlers that must not > > run > > in nested event loops). I didn't find an elegant way preserve that > > behavior, so > > I added a new API called qio_channel_set_follow_coroutine_ctx(ioc, > > true|false) > > for opting in to the new AioContext model. By default QIOChannel uses the > > iohandler AioHandler. Code that formerly called > > qio_channel_attach_aio_context() now calls > > qio_channel_set_follow_coroutine_ctx(ioc, true) once after the QIOChannel is > > created. > > > > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > > --- > > +++ b/include/io/channel-util.h > > @@ -49,4 +49,27 @@ > > QIOChannel *qio_channel_new_fd(int fd, > > Error **errp); > > > > +/** > > + * qio_channel_yield: > > Wrong function name... > > > + * @read_fd: the file descriptor for the read handler > > + * @read_ctx: the AioContext for the read handler > > + * @io_read: the read handler > > + * @write_fd: the file descriptor for the write handler > > + * @write_ctx: the AioContext for the write handler > > + * @io_write: the write handler > > + * @opaque: the opaque argument to the read and write handler > > + * > > + * Set the read and write handlers when @read_ctx and @write_ctx are > > non-NULL, > > + * respectively. To leave a handler in its current state, pass a NULL > > + * AioContext. To clear a handler, pass a non-NULL AioContext and a NULL > > + * handler. > > + */ > > +void qio_channel_util_set_aio_fd_handler(int read_fd, > > ...should be this. > > > + AioContext *read_ctx, > > + IOHandler *io_read, > > + int write_fd, > > + AioContext *write_ctx, > > + IOHandler *io_write, > > + void *opaque); > > + > > #endif /* QIO_CHANNEL_UTIL_H */ > > diff --git a/include/io/channel.h b/include/io/channel.h > > index 229bf36910..5f9dbaab65 100644 > > --- a/include/io/channel.h > > > +++ b/io/channel.c > > > > > -static void qio_channel_set_aio_fd_handlers(QIOChannel *ioc) > > +static void coroutine_fn > > +qio_channel_set_fd_handlers(QIOChannel *ioc, GIOCondition condition) > > { > > - IOHandler *rd_handler = NULL, *wr_handler = NULL; > > + AioContext *ctx = ioc->follow_coroutine_ctx ? > > + qemu_coroutine_get_aio_context(qemu_coroutine_self()) : > > + iohandler_get_aio_context(); > > + AioContext *read_ctx = NULL; > > + IOHandler *io_read = NULL; > > + AioContext *write_ctx = NULL; > > + IOHandler *io_write = NULL; > > + > > + if (condition == G_IO_IN) { > > + ioc->read_coroutine = qemu_coroutine_self(); > > + ioc->read_ctx = ctx; > > + read_ctx = ctx; > > + io_read = qio_channel_restart_read; > > + > > + /* > > + * Thread safety: if the other coroutine is set and its AioContext > > + * match ours, then there is mutual exclusion between read and > > write > > matches > > > + * because they share a single thread and it's safe to set both > > read > > + * and write fd handlers here. If the AioContext does not match > > ours, > > + * then both threads may run in parallel but there is no shared > > state > > + * to worry about. > > + */ > > + if (ioc->write_coroutine && ioc->write_ctx == ctx) { > > + write_ctx = ctx; > > + io_write = qio_channel_restart_write; > > + } > > + } else if (condition == G_IO_OUT) { > > + ioc->write_coroutine = qemu_coroutine_self(); > > + ioc->write_ctx = ctx; > > + write_ctx = ctx; > > + io_write = qio_channel_restart_write; > > + if (ioc->read_coroutine && ioc->read_ctx == ctx) { > > + read_ctx = ctx; > > + io_read = qio_channel_restart_read; > > + } > > + } else { > > + abort(); > > + } > > + > > + qio_channel_set_aio_fd_handler(ioc, read_ctx, io_read, > > + write_ctx, io_write, ioc); > > +} > > + > > With those minor fixes, > > Reviewed-by: Eric Blake <ebl...@redhat.com>
Acked-by: Daniel P. Berrangé <berra...@redhat.com> Eric, feel free to merge the two io subsystem changes at the same time as the NBD changes. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|